The following application is related to the present application and its disclosure is incorporated herein by reference:
U.S. Utility patent application Ser. No. 09/620,446 entitled xe2x80x9cSystem and Method for Charging A Capacitor Using a Constant Frequency Current Waveform,xe2x80x9d and naming as inventor Gregory D. Brink.
1. Field of the Invention
The present invention relates generally to charging capacitors and, more particularly, to a method and apparatus for charging high voltage capacitors.
2. Related Art
Sudden cardiac arrest has been attributed to over 350,000 deaths each year in the United States, making it one of the country""s leading medical emergencies. Worldwide, sudden cardiac arrest has been attributed to a much larger number of deaths each year. One of the most common and life threatening consequences of a heart attack is the development of a cardiac arrhythmia, commonly referred to as ventricular fibrillation. When in ventricular fibrillation, the heart muscle is unable to pump a sufficient volume of blood to the body and brain. The lack of blood and oxygen to the brain may result in brain damage, paralysis or death to the victim.
The probability of surviving a heart attack or other serious heart arrhythmia depends on the speed with which effective medical treatment is provided. If prompt cardiopulmonary resuscitation is followed by defibrillation within approximately four minutes of the onset of symptoms, the probability of survival can approach or exceed fifty percent. Prompt administration of defibrillation within the first critical minutes is, therefore, considered one of the most important components of emergency medical treatment for preventing death from sudden cardiac arrest.
Cardiac defibrillation is an electric shock that is used to arrest the chaotic cardiac contractions that occur during ventricular fibrillation, and to restore a normal cardiac rhythm. To administer such an electrical shock to the heart, defibrillator pads are placed on the victim""s chest, and an electrical impulse of the proper magnitude and shape is administered to the victim through the pads. While defibrillators have been known for years, they have typically been complicated, making them suitable for use by trained personnel only.
More recently, portable and transportable automatic and semi-automatic external defibrillators (generally, AEDs) for use by first responders have been developed. A portable defibrillator allows proper medical care to be given to a victim earlier than preceding defibrillators, increasing the likelihood of survival. Such portable defibrillators may be brought to or stored in an accessible location at a business, home, aircraft or the like, available for use by first responders. With recent advances in technology, even a minimally trained individual can operate conventional portable defibrillators to aid a victim in the critical first few minutes subsequent to the onset of sudden cardiac arrest.
As noted, effective medical treatment must be administered promptly after the onset of symptoms. One time consuming defibrillator operation is the charging of a high voltage capacitor that provides the energy for producing the electric shock. Unfortunately, conventional AEDs do not efficiently charge the high voltage capacitor, consuming valuable time preparing to provide the therapy. This limits the number of multiple shocks that can be administered to a patient in the minimal time available. What is needed, therefore, is a defibrillator that can charge a high voltage capacitor quickly and efficiently.
The present invention is a system and method for charging a high-voltage capacitor through the application of a current, the magnitude of which has a variable frequency, variable duty cycle waveform. Generally, energy is transferred from a power source to the high voltage capacitor via a magnetic element such as an inductor or transformer. For example, a pulsed voltage supply provides voltage pulses having a variable frequency and an adjustable duty cycle to a primary winding of a fly-back transformer.
During a charging sequence in which current charge cycles are applied to the capacitor, the duty cycle of the variable frequency current waveform is controlled dynamically based on the rate at which energy can be transferred to the capacitor. Specifically, during a charge sequence, current pulses through the primary winding are controlled such that the transformer operates in a continuous mode during an initial portion of the charge sequence, and, to the extent necessary to fully charge the capacitor, in a discontinuous mode during a subsequent portion of the charge sequence. During the continuous mode, the duration of the off or non-conduction time of the primary winding current waveform is limited to be less than or equal to a maximum time period. Within this maximum time period energy can be transferred efficiently from the transformer to the capacitor. Subsequent, inefficient conduction time is inhibited. This increases the speed at which the high voltage capacitor is charged.
A number of aspects of the invention are summarized below, along with different embodiments that may be implemented for each of the summarized aspects. It should be understood that the summarized embodiments are not necessarily inclusive or exclusive of each other and may be combined in any manner in connection with the same or different aspects that is non-conflicting and otherwise possible. These disclosed aspects of the invention, which are directed primarily to high performance capacitor charging systems and methodologies, are exemplary aspects only and are also to be considered non-limiting.
In one aspect of the invention, a capacitor charging system connected electrically to a high voltage capacitor is disclosed. The capacitor charging system is constructed and arranged to charge the capacitor by generating a variable frequency, variable duty cycle current waveform. In one embodiment, the system generates a charge sequence of successive current charge cycles, and includes a transformer and a controller. The transformer has a primary winding connected in series to a voltage source and a secondary winding across which the capacitor is electrically coupled. The controller controls a primary current through the primary winding such that during each of a first plurality of charge cycles the transformer does not transfer all stored energy to the capacitor, and during each of a subsequent plurality of charge cycles the transformer transfers substantially all stored energy to the capacitor. In one implementation, the transformer may be a fly-back transformer. In such an implementation the controller can include two current detectors. A first current detector determines when the primary current achieves a maximum current while the second current detector determines when current flowing through the secondary winding falls below a minimum current. The controller controls the primary current based at least in part on these conditions of the primary and secondary winding current.
In another aspect of the invention a system for charging a high-voltage capacitor is disclosed. The system includes a fly-back transformer having a primary winding, a core and a secondary winding that is out of phase with the primary winding. The capacitor is electrically coupled across the secondary winding. A charge controller is also included. The charge controller applies a charge sequence of successive current charge cycles to the primary winding. Each charge cycle has an on time and an off time. For each charge cycle on time the controller enables the primary current to flow through the primary winding until the primary current reaches a maximum current. For each charge cycle off time the controller inhibits the primary current until either current through the secondary winding is approximately zero or a maximum charge cycle off time has transpired. In operation, for a first plurality of charge cycles the maximum off time transpires prior to the secondary winding current becoming approximately zero, resulting in the transformer storing energy during each of the first plurality of charge cycles. For a second plurality of charge cycles occurring subsequent to the first plurality of charge cycles, the secondary winding current becomes zero prior to the maximum off time transpiring, resulting in the transformer transferring substantially all stored energy to the capacitor during the second plurality of charge cycles.
In a further aspect, a high-voltage capacitor charging system is disclosed. The system generates current pulses having a variable frequency to charge a capacitor. During a charging sequence in which the current pulses are repeatedly applied to the capacitor, the duty cycle of the variable frequency current waveform is controlled dynamically based on the rate at which energy can be transferred to the high voltage capacitor.
In a still further aspect of the invention a method for charging a capacitor is disclosed. The method includes the steps of providing a fly-back transformer having a primary winding, a core and a secondary winding that is out of phase with the primary winding; and applying a charge sequence of successive current charge cycles to the primary winding. The latter step is performed such that for each charge cycle on time the primary current is applied until the primary current reaches a maximum current, and for each charge cycle off time the primary current is inhibited until either current through the secondary winding is approximately zero or a maximum charge cycle off time has transpired.
In one embodiment, the maximum charge cycle off time is of a duration such that for a first plurality of charge cycles the maximum off time transpires prior to the secondary winding current becomes approximately zero, resulting in the transformer storing energy during each of the first plurality of charge cycles, and for a second plurality of charge cycles occurring subsequent to the first plurality of charge cycles, the secondary winding current becomes zero prior to the maximum off time transpiring, resulting in the transformer transferring substantially all stored energy to the capacitor during the second plurality of charge cycles.
Various embodiments of the present invention provide certain advantages and overcome certain drawbacks of the conventional techniques. Not all embodiments of the invention share the same advantages and those that do may not share them under all circumstances. This being said, the present invention provides numerous advantages including the noted advantage of efficiently transferring energy to a high voltage capacitor. Alternative or additional significant benefits may be realized depending on the desired application. For example, systems implementing the present invention can provide charge times comparable to conventional systems using smaller components, a lower energy power source, a higher impedance power source or any reasonable combination thereof. These and other features and advantages of the present invention as well as the structure and operation of various embodiments of the present invention are described in detail below with reference to the accompanying drawings.